Mechanical Property Testing of Spark Surfacing Weld Joints

Literature Overview

This 2008 study by Zheng Xiangfeng, Feng Yanting, Li Zhongwei, and Wang Qing from the Hebei Electric Power Research Institute investigates the mechanical properties of weld joints produced by electric spark surfacing (ESW) technology. Funded by Hebei Electric Power Company (project DK2008-33), the research addresses a critical gap in the qualification and acceptance of ESW overlay welds used in power generation equipment. The work was published in "Hot Working Technology" and represents a significant contribution to the understanding of ESW weld quality assessment.

Core Technical Concepts

Electric spark surfacing (ESW) is a solid-state bonding process that uses electrical discharge to join a cladding strip to a base metal substrate. Unlike fusion welding processes, ESW does not melt the base metal, resulting in minimal dilution and a metallurgical bond formed through plastic deformation and mechanical interlocking. This makes ESW particularly attractive for applications requiring a sharp interface between dissimilar materials, such as stainless steel overlay on carbon steel for corrosion resistance.

ESW Process Fundamentals

The ESW process operates by passing a high-current electrical discharge through a copper electrode that contacts a cladding strip and the base metal simultaneously. The discharge generates localized heating that causes plastic deformation of both the strip and the base metal surface, creating a mechanical and metallurgical bond. The process parameters include discharge current (typically 1500-4000 A), discharge frequency (10-50 Hz), strip feed speed (0.5-5 m/min), and electrode pressure (50-200 N).

Mechanical Property Assessment

The study systematically evaluates the mechanical properties of ESW weld joints through tensile testing, shear testing, hardness profiling, and microstructural analysis. The key findings reveal that the bond strength of ESW joints is primarily governed by the quality of the mechanical interlocking at the interface, which is determined by the degree of plastic deformation achieved during the discharge process.

The tensile test results show that ESW joints typically achieve bond strengths in the range of 200-400 MPa for stainless steel strip on carbon steel substrate, which is generally lower than fusion-welded joints but sufficient for most corrosion overlay applications where the primary function of the overlay is chemical resistance rather than structural load-bearing.

Process Parameters and Their Influence

Effect of Discharge Current

The discharge current is the most critical parameter governing bond quality. Insufficient current results in inadequate plastic deformation and weak bonding, while excessive current can cause localized melting, oxidation, and even burn-through of thin strips. The optimal current density at the contact area should be in the range of 10-50 A/mm² to achieve full plastic deformation without melting.

Effect of Feed Speed

Feed speed directly affects the number of discharge pulses per unit length of strip, which in turn determines the uniformity of bonding along the strip length. Too high a feed speed results in insufficient bonding intervals and weak spots, while too low a speed causes excessive heat accumulation and potential strip overheating. The optimal feed speed is typically in the range of 1.5-3.0 m/min for standard strip thicknesses of 0.5-1.0 mm.

Effect of Electrode Pressure

Electrode pressure determines the contact resistance and the intensity of the discharge. Insufficient pressure leads to unstable arcing and poor bonding, while excessive pressure can cause electrode wear and strip damage. The optimal pressure range of 100-150 N provides stable discharge and consistent bonding quality.

Defect Analysis and Quality Control

Common Defects in ESW Joints

Quality Control Procedures

A comprehensive quality control program for ESW welds should include the following steps:

1. Visual inspection - Check for uniform strip coverage, absence of burn-through, and proper edge alignment
2. Magnetic particle testing (MT) - Detect surface and near-surface cracks in the strip and at the interface
3. Penetrant testing (PT) - Identify surface discontinuities and unbonded areas
4. Ultrasonic testing (UT) - Evaluate bond quality and detect subsurface defects
5. Shear testing - Quantify bond strength according to ASTM A263 or equivalent standards
6. Hardness profiling - Verify hardness distribution across the strip and base metal
7. Metallographic examination - Assess interface microstructure and bonding quality

Engineering Practice and Application

Application in Power Generation

The primary application of ESW in the power generation industry is the overlay of stainless steel or nickel-based alloy strips on carbon steel or low-alloy steel components to provide corrosion resistance. Typical applications include:

• Boiler tubes: Overlay of 310SS or 347H stainless steel on T/P91 tubes for improved high-temperature corrosion resistance
• Heat exchanger tubes: Overlay of 316L or Inconel 625 on carbon steel tubes for improved resistance to flue gas desulfurization (FGD) media
• Pressure vessels: Overlay of 304L stainless steel on SA-516 Gr.70 vessels for improved resistance to corrosive process media
• Piping systems: Overlay of 316L on carbon steel piping for improved resistance to chloride-induced stress corrosion cracking

Qualification Requirements

According to NB/T 47014 and ASME IX, ESW welds must be qualified through procedure qualification and welder performance qualification. The procedure qualification test coupon must demonstrate adequate bond strength (minimum 200 MPa for shear testing per ASTM A263), acceptable hardness in the heat-affected zone (HAZ), and sound microstructure. The welder performance qualification requires demonstration of the ability to produce sound welds under production conditions.

Key Questions and Reflections

One of the most significant findings from this research is the recognition that ESW bond strength is not solely determined by the metallurgical bond but is also strongly influenced by the mechanical interlocking achieved through plastic deformation. This insight has important implications for process parameter selection and quality control. The degree of plastic deformation at the interface can be controlled by adjusting the discharge current, electrode pressure, and feed speed, and these parameters must be optimized for each specific application.

Another important consideration is the effect of base metal surface preparation on bond quality. The base metal surface must be clean, free of oxide, and have adequate surface roughness to promote mechanical interlocking. Surface preparation methods include grinding, shot blasting, and chemical cleaning, and the selection of the appropriate method depends on the base metal material and the service conditions.

Study Insights and Implications

The work by Zheng and colleagues provides valuable guidance on the mechanical property assessment of ESW welds and highlights the importance of systematic quality control in ensuring reliable bond quality. The research demonstrates that ESW is a viable alternative to fusion welding for corrosion overlay applications, particularly where minimal dilution and a sharp interface are required.

For engineers involved in specifying and qualifying ESW welds, this work emphasizes the need for comprehensive mechanical property testing and the importance of understanding the relationship between process parameters and bond quality. The research also highlights the challenges associated with ESW quality control, particularly the difficulty of detecting unbonded areas and the need for multiple testing methods to ensure comprehensive defect detection.